The dependency output for header unit modules is based on the absolute
pathname of the header file, but that's not something that a makefile can
portably refer to.  This patch adds a .c++-header-unit target based on the
header name relative to an element of the include path.

libcpp/ChangeLog:

	* internal.h (_cpp_get_file_dir): Declare.
	* files.cc (_cpp_get_file_dir): New fn.
	* mkdeps.cc (make_write): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/dep-4.H: New test.